Fabrication of high-quality multilayer structure for HTS-SFQ circuits using surface treatments

Surface treatments have been applied to a multilayer structure in order to improve its surface smoothness and crystallinity for fabricating HTS-SFQ circuits. The surface treatments consist of ion beam etching and annealing in oxygen atmosphere. These surface treatments were applied to a SrSnO₃ insulating layer on a La-YBCO grandplane as an underlayer for growth of a La-YBCO base layer. From RHEED observation, it was found that a contamination layer or an amorphous layer formed on the SrSnO₃ surface after the bilayer was exposed to oxygen plasma for removing a photoresist film. The SrSnO₃ surface structure was observed to be recovered through the ion etching and subsequent annealing procedure. XRD measurement revealed that FWHM of the SrSnO₃ [200] rocking curve reduced from that of the as-deposited SrSnO₃ after an annealing at 715°C in 120 mTorr oxygen atmosphere, indicating that crystallinity of the SrSnO₃ layer was also slightly improved. The recovery process for the SrSnO₃ layer significantly improved the surface roughness Ra and the J_c at 4.2 K of the La-YBCO base layer.